1. Field of the Invention
The present invention relates to a process for producing a semiconductor device. More particularly, the present invention relates to a process for producing a silicon gate field effect semiconductor device.
2. Description of the Prior Art
A conventional silicon gate field effect semiconductor device is provided with a gate electrode, a source region, a drain region and an insulating layer for insulating the gate electrode from the source and drain regions. The gate electrode is prepared by forming a polycrystalline silicon layer on a gate insulating layer and by diffusing an impurity substance, such as arsenic or phosphorus, into the polycrystalline silicon layer to convert it into an electroconductive layer having a very small resistivity. Also, the source and drain regions are prepared in a single crystal silicon layer on which the insulating layer and the gate electrode are supported, by diffusing the impurity substance into portions of the single crystal silicon substrate in accordance with a predetermined pattern.
In the conventional semiconductor device which works at a low speed, the source and drain regions have a relatively large depth, for example, 9000 to 30000A, respectively. When the above-mentioned low speed type of semiconductor device is prepared, in accordance with a conventional process, a material which is capable of generating therefrom the impurity substance to be diffused is applied onto the surfaces of the polycrystalline silicon layer and the single crystal silicon substrate in accordance with the predetermined pattern. Then, the impurity substance-generating material layer is heated in an oxygen-containing atmosphere at an elevated temperature of, for example from 900.degree. to 1100.degree. C., so as to allow the resultant impurity substance to diffuse into both the polycrystalline silicon layer and the single crystal silicon substrate. In this solid-to-solid type of diffusing operation, it is known that the diffusion speed of the impurity substance in the single crystal silicon substrate is larger than that in the polycrystalline silicon layer. However, in the preparation of the low speed type of semiconductor device, since the depth of the source and drain regions is large, when the diffusing operation is carried out over such a period of time that the impurity substance can thoroughly diffuse into the single crystal silicon substrate up to a large depth thereof, so as to provide source and drain regions each having a large depth, the impurity substance also can thoroughly diffuse into the polycrystalline silicon layer, so as to completely convert it into the electroconductive gate electrode.
However, recently, there has developed a requirement to provide a new type of semiconductor device which has a relatively small size and which works at a relatively high speed. In this type of the semiconductor device, the source and drain regions are required to have a relatively small depth, for example, 1500 to 4500A, respectively. If the above-mentioned conventional method is applied to produce the new type of the semiconductor device, the diffusion operation should be limited to a relatively short time, so as to cause the resultant source and drain regions to have a relatively small depth, respectively. However, under this condition, the impurity substance can not thoroughly diffuse into the polycrystalline silicon layer. Therefore, the resultant silicon gate electrode has a poor electroconductivity, and is useless. The disadvantages of the conventional process will be explained again in more detail hereinafter.
In another conventional process, the impurity substance is diffused into both the polycrystalline silicon layer and the single crystal silicon substrate in accordance with an ion implantation method. In this method, the impurity substance is dosed in the same amount to the polycrystalline silicon layer as that to the single crystal silicon substrate. In the case of the high speed type semiconductor device, the source and drain regions have a small depth, respectively. The amount of the impurity substance sufficient for providing the source and drain regions is small. Accordingly, the amount of the impurity substance dosed to the polycrystalline silicon layer, which is the same as that dosed to the single crystal silicon substrate, is also small and, therefore, is insufficient for converting the polycrystalline silicon layer to the electroconductive gate electrode.
In order to increase the electroconductivity (and decrease the resistivity) of the polycrystalline silicon layer incompletely diffused by the impurity substance, it is necessary to apply an additional diffusing operation of the impurity substance into the incompletely diffused polycrystalline silicon layer. This necessity causes the process to be complicated and expensive. Otherwise, the diffusing operation of the impurity substance to the polycrystalline silicon layer is carried out separately from that to the single crystal silicon substrate. In this case, an insulating layer including a gate insulating portion and a field insulating portion, is formed on a substrate of single crystal silicon, and then a polycrystalline silicon layer is formed on the gate insulating portion. A first diffusing operation of the impurity substance is applied to the polycrystalline silicon layer. During this operation, a sufficient amount of the impurity substance for obtaining an electroconductive gate electrode diffuses into the polycrystalline silicon layer, whereas no impurity substance can diffuse into the single crystal silicon layer due to the barrier composed of the insulating layer. In accordance with a predetermined pattern, the field insulating portion is etched to open windows on the single crystal silicon substrate. A second diffusing operation of the impurity substance is applied to the opened portions of the single crystal silicon substrate for forming the source and drain regions therein. The above-mentioned process is effective for obtaining the electroconductive gate electrode, and the source and drain regions each having a desired quality. However, this process is complicated and expensive.